The present disclosure herein relates to a terminally-crosslinked methyl morpholinium-functionalized block copolymer, and an anion exchange membrane using the same, and more particularly, to a terminally-crosslinked block copolymer which has a novel structure, and in which, in a poly(arylene ether sulfone) multiblock copolymer (MM-PES) having methyl morpholinium as a conducting group, an azide compound may be used as a crosslinking agent so that crosslinking only occurs at ends of the polymer chains (xMM-PES), thereby minimizing conductivity loss, significantly increasing mechanical and chemical stability, attaining additional conductivity resulting from the three-dimensional structure of morpholinium, and reducing water uptake while enhancing water retention capacity; uses thereof as an alkaline fuel cell anion exchange membrane (AEM); and a method for conveniently preparing the same through simple heat-treatment.
Anion exchange membranes (AEMs), due to the possibility of application to electrochemical energy conversion/storage devices, such as fuel cells, electrodialysis cells, and redox flow cells, has received significant attention over the past 10 years. When AEMs are used in such devices, costs may be reduced because non-noble metal catalysts may be used through operation in alkaline conditions.
However, the poor stability of AEMs in highly alkaline (high pH) operating conditions, particularly at high temperatures (>80° C.), is limiting the successful adoption of such techniques.
Accordingly, there has been much effort in recent years focusing on improving the chemical stability of polymer backbones and anionic (OH−) conducting groups in AEMs. Various polymer backbone structures, including poly(arylene ether sulfone), poly(phenylene oxide), poly(olefin), poly(styrene), and poly(phenylene), have been examined as AEMs.
However, there is not yet a consensus of opinions on what is the most promising polymer structure.
Crosslinking enhances the alkaline stability of AEM polymer backbones, and thus may be an efficient method for achieving long-term stability in a membrane.
However, most crosslinking systems necessarily entail a reduction in ionic conductivity because the formation of a rigid 3D-network and the resulting low water uptake suppresses ionic conduction in crosslinking networks.
Accordingly, several different approaches have been attempted, such as blending or ionic conductor-mediated crosslinking
Meanwhile, various research is being carried out to improve the alkaline stability of AEM conductive groups. Cations, including the most typically used quaternary ammonium (QA), may be introduced into AEMs, and examples thereof may include imidazolium, guanidinium, phosphonium, sulfonium, and metal-based hybrid cations and the like.
Recently, aliphatic heterocyclic QA cationic head groups such as piperidinium and morpholinium have been considered the most stable due to such aliphatic heterocyclic QA cationic head groups having bulky structures, and such bulky structures impede the approach of OH− to polymers connected with anion-conductive groups.
Another advantage of such conductors is enhanced ionic conductivity, which is due to the high ionic dissociation properties of the conductors resulting from the bulky structures thereof.
Therefore, polymers functionalized by n-methyl piperidinium and n-methyl morpholinium may exhibit excellent alkaline stability and high OH− conductivity.
The present inventors have developed a terminally-crosslinked sulfonated ionomer (ion-conductive polymer) using an azide as a proton exchange membrane. Unlike most crosslinked polymer systems, such sulfonated ionomers are were only crosslinked at polymer chain ends.
A membrane manufactured from a terminally-crosslinked sulfonated poly(arylene ether sulfone) (sPES) was able to minimize conductivity loss following crosslinking by minimizing the formation of a 3-D crosslinking network, and most of the structural perfection was maintained.
Moreover, such a terminally-crosslinked membrane exhibited particularly high proton conductivity, in particular under high temperature and/or partially hydrated relative humidity (RH) conditions. This can be explained by the formation of a unique terminally-crosslinked system in which more water is trapped inside a 3-D network due to structural reorganization of a polymer backbone induced by hydrothermal energy.